1. Field of the Invention
The present invention relates generally to intraocular lenses, and more specifically to intraocular lenses containing markings.
2. Description of the Related Art
Intraocular lenses are used replace or supplement the natural lens of an eye. Similar to other ophthalmic lenses, such as spectacles and contact lenses, intraocular lenses may be configured to provide both spherical and cylinder power. The cylinder power is used to correct rotational asymmetry astigmatism of the cornea or eye of a subject that degrades the vision of the eye and cannot be corrected by adjusting the spherical power of the lens. Lenses that are configured to correct astigmatism are commonly referred to as toric lenses.
The astigmatism of an eye or cornea may be characterized in terms of a magnitude in Diopters along an astigmatic axis. To correct this astigmatism, or at least reduce its magnitude, a toric lens is provided. As used herein, a toric lens is characterized by a base spherical power (which may be positive, negative, or zero) and a cylinder power that is added to the base spherical power of the lens for correcting the astigmatism of the eye. As used herein, a toric lens is also characterized by a “low power meridian” with a constant power equal to the base spherical power and an orthogonal “high power meridian” with a constant power equal to the base spherical power plus the cylinder power of the lens, To correct astigmatism of the eye, an angular orientation of the astigmatic axis of the cornea or eye is first determined, then a toric lens with a predetermined base spherical power and cylinder power is aligned so that the low power meridian of the toric lens is at the same angle as the astigmatic axis of the cornea or eye. When correct alignment between the axes of the lens and cornea is achieved, the astigmatism of the lens may cancel, or at least significantly reduce, the astigmatism of the cornea, resulting in improved vision over an intraocular lens having spherical correction only. While correct angular alignment between the astigmatic axis of the cornea and the low power meridian of the corrective lens is generally important for providing optimal correction, alignment is particular critical in the case of intraocular lenses, since it is much more difficult to replace or re-orient an intraocular lens once it has been placed into the eye.
In practice, there may be some residual astigmatism left in the eye due to a rotational misalignment between the astigmatic axis of the cornea and the low power meridian of the corrective intraocular lens. Mathematically, the astigmatism of the cornea (amount−A, orientation θ), plus an astigmatism of the rotationally misaligned lens (amount+A, orientation θ+δ), results in a residual astigmatism with magnitude 2A sin δ, oriented at 45° to the angle (θ+δ/2). As an example, consider a cornea that has 2 Diopters of astigmatism, and a lens that has 2 Diopters of cylinder power. If the lens is implanted with an angular error δ of 5 degrees, then the residual astigmatism is (2)(2 Diopters)(sin 5°)=0.35 Diopters. For a looser tolerance of 10 degrees, the residual astigmatism is (2)(2 Diopters)(sin 10°)=0.7 Diopters. A typical threshold for astigmatism is 0.25 Diopters, so that if the light reaching the retina has less than 0.25 Diopters of astigmatism, then the astigmatism does not significantly degrade the vision of the eye.
Accordingly, there is a need for toric intraocular lenses and methods of implanting such lenses that aid a surgeon in quickly and accurately aligning such lenses with the astigmatic axis of the cornea.